rodent, Any member of the order Rodentia, which contains 50% of all living mammal species. Rodents are gnawing, mostly herbivorous, placental mammals. They have one pair of upper and one pair of lower, continuously growing, incisors. When the lower jaw is pulled back, the cheek teeth connect for grinding; when it is pulled forward and down, the incisors meet at the tips for gnawing. Rodent families include squirrels (Sciuridae); Old World mice (see mouse) and rats (Muridae); deer mice (see deer mouse), gerbils, hamsters, lemmings, muskrats, wood rats, and voles (Cricetidae); beaver (Castoridae); gophers (Geomyidae); guinea pigs (Caviidae); pocket mice (see pocket mouse) and kangaroo rats and mice (Heteromyidae); New and Old World porcupines (Erethizontidae and Hystricidae); and hutia (Capromyidae).